Comment by gpderetta

The primary reason you might want reference stability is because you might have another pointer to it. Reference stability is generally important for the STL and carefully documented; even for vectors stability is guaranteed on many modify operations. I have seen plenty of code that use reserve+push_back specifically to preserve stability for example.

The primary reason for unordered_map to preserve stability is that std::map does and the ability of unordered_map to be a drop-in replacement for std::map for code that doesn't need ordering was considered at that time a better tradeoff than the absolute optimal implementation.

You can of course build a pointer stable map on top of an unstable one by heap allocating every object and storing unique_ptrs to the objects in the map, but when hash_map was initially added to the STL (remember than unordered map is just standardizing the non-standard hash_map), there was no unique_ptr (we do not speak of auto_ptr).